1H Nuclear magnetic resonance (NMR) relaxometry was exploited to investigate the dynamics of solid proteins. The relaxation experiments were performed at 37 °C over a broad frequency range, from approximately 10 kHz to 40 MHz. Two relaxation contributions to the overall 1H spin-lattice relaxation were revealed; they were associated with 1H-1H and 1H-14N magnetic dipole-dipole interactions, respectively. The 1H-1H relaxation contribution was interpreted in terms of three dynamical processes occurring on timescales of 10-6 s, 10-7 s, and 10-8 s, respectively. The 1H-14N relaxation contribution shows quadrupole relaxation enhancement effects. A thorough analysis of the data was performed revealing similarities in the protein dynamics, despite their different structures. Among several parameters characterizing the protein dynamics and structure (e.g., electric field gradient tensor at the position of 14N nuclei), the orientation of the 1H-14N dipole-dipole axis, with respect to the principal axis system of the electric field gradient, was determined, showing that, for lysozyme, it was considerably different than for the other proteins. Moreover, the validity range of a closed form expression describing the 1H-14N relaxation contribution was determined by a comparison with a general approach based on the stochastic Liouville equation.
Keywords: NMR relaxometry; dynamics; proteins; quadrupole relaxation enhancement; relaxation; solids.